Tinned copper radiator fin



Feb. 21, 1939. 1 M. LAWTON ET AL TINNED COPPER RADIATOR FTN Filed June ll, 1937 /-COPPER COPPER BRDNZE gmc/rm PHOSPHOR- CDPPER WIR SHV/Mu Patented Feb. 21, 1939 UNITED STATES PATENT OFFICE TTNNED COPPER AmA'ron FIN Application June 11, 1937, Serial No. 147,616

6 Claims.

In the manufacture of heat exchangers of the fin and tube type the core is often made from a series of preformed copper plates stacked together in complementary relation and sealed at the joints between succeeding plates by dipping the core faces in molten solder. Such cores are not well adapted for installations where internal pressures arev high and in the development of a stronger core it has been proposed to braze the joints by placing the assembly in a furnace with suitable fusible material laid adjacent the joints, to be melted for flow around and between the abutting surfaces of the joints. The fusible material selected consists of phosphorcopper wire. containing from 4% to 6% phosphorus which melts at about 1325 F. and is free flowing at1400 F. so that the brazing furnace is run at about 1450" F. Such temperature is above the'annealing temperature of copper. Consequently, while an excellent joint is had the thin copper used is rendered extremely soft.

It is an object of the present invention to make use of the brazing temperature for stiiening rather than softening the core plates so that the assembly is made even more rigid. To this end the thin copper plates are coated with tin to an amount such that a portion of the copper is changed into bronze during the heating period of the brazing operation. The bronze skin so formed not only strengthens the core assembly but in addition protects against ccrrosion and does not materially change the heat transfer properties of the core. By comparative tests the bursting strength of the tubes is raised about 37%.

A more detailed explanation will be given in connection with the accompanying drawing wherein Figure l is an elevation of a fragment of a known type of radiator core with which the invention is adapted for application; Figure 2 is an end view of the core laid in a horizontal position on one face appropriate for the brazing operation; and Figures 3 and 4 illustrate on a greatly enlarged scale a section of a core plate before and after alloying has taken lace. p In compliance with the patent act only the best known mode of applying the invention will be herein disclosed, but this is not to be taken as a limitation of scope.

The core shown for illustration consists of a numberof plates I each having pressed out at symmetrically spaced intervals a number of tube formations or hollow elongated projections 2 which when nested together form a series of spaced passages or liquid iiow tubes with the undisturbed portions of the plates constituting heat radiating fins for the flow of air thereover. The individual tube plates are formed from ribbon stock passed through suitable dies and cut to length. Because of its good conductivity, copper is the metal chosen for the core plates and the thickness of the copper stock usually is in the neighborhood of 0.0045 inch thick. This is easily worked and the pressing of the formations therein serves to strengthen the metal. Either before or after Working a thin coating of tin is applied by electroplating, dipping or the like to both faces of the copper ribbon as illustrated in Figure 3. This envelope of tin ultimately alloys with the ,adjacent portion of the copper to form a bronze skin illustrated in Figure 4. For proper conversion, at the temperatures involved, the layer of tin must be controlled and held within certain limits and tests indicate that for the thickness of copper mentioned the tin covering on each face should not exceed 0.00025 inch, which means that the tin is approximately 10% by volume of the coated copper plate. Too thick a tin coating produces a bronze alloy whose melting point is below the bracing temperatures tobe used and, therefore, lis impractical,

After assembling the tin copper stripsthe core is placed in a flat position within an oven or furnace and there are laid on top in line with the several Water tubes lengths of phosphorcopper material conveniently in the form of wires as shown by broken lines at 3 in Figures l and 2. The oven temperature is maintained in the neighborhood of 1450 F. and as the assembly heats up the Wires 3 containing phosphorus to an amount of about 4% to 6% melts and flows downwardly into all the joints between the nested tube projections 2 so as to seal and bond the tube passages. Concurrently there is a surface diffusion into the copper of the tin coating and the two metals combine forming a bronze skin which extends for approximately 0.001 inch under the surface on both sides of the core plate. Due to this alloying the surface is hardened and the plates are stiiened materially so as better to withstand crushing shock as well as bursting pressures. 'I'he hardened skin also resists corrosion.

In regard to the stiilening or hardening effect of the process, it is only in relatively thin structures that the process has any value, but because of the corrosion resistant properties of the bronze skin as compared to bare copper.. it may be found useful in other situations.

To avoid the possibility of loss of tin by oxidation during the application of heat and before an alloying temperature is reached, it is proposedto carry on the heating step in a controlled nonoxidizing atmosphere. A conventional hydrogen brazing oven may be used for this purpose.

Thus it will be seen that according to the above description the core plates are formed in the soft metal and are subsequently hardened and simultaneously bonded together in the brazing furnace. The rigidity and corrosion resistance of a core made in this fashion renders it especially adapted for use with pressure systems, steam heating plants and as oil coolers, condensers for refrigerators and the like.

We claim:

1. The method of bonding together a series of preformed and relatively thin plates and simul taneously stiffening the same, comprising coating a copper ribbon of about 0.0045 inch thick with tin on both faces to a depth of about 0.00025 inch, working the ribbon to shape in the soft metal, nesting together a. group of such ribbons, placing adjacent the contacting surfaces to be joined a wire of phosphor-copper containing approximately 4% to 6% phosphorus and being free'ilowing at about 1400 F., and then heating the parts to a temperature of about 1450 F'., which melts and flows the material of the wire to said contacting surfaces and also alloys the tin and copper into a stiff bronze skin.

2. The method of forming a spaced plate assembly comprising applying a skin coating of tin to the opposite faces of a relatively thin and soft copper ribbon, working complementary plates from the tinned copper ribbon, stacking the formed plates in assembly relation, and then subjecting the assembly to an alloying temperature to diffuse the copper into the tin and thereby stiifen the plates, the heat being continued until all the tin becomes alloyed with the copper.

3. The method of forming a spaced plate assembly comprising stacking together a number of pre.

formed and tin coated complementary copper plates, placing fusible material adjacent joints to be sealed and then applying heat to the assembly to melt and iiow said material to said joints and concurrently convert the tin and copper into a bronze skin, the heat being continued until all the tin becomes alloyed with the copper.

4. In the manufacture of radiator cores having a series of spaced tubes and heat radiating fins between adjacent tubes, the method comprising preforming and stacking in the conventional manner, a series of mating tin coated copper plates, placing adjacent each tube to be sealed a phosphor-copper wire, and then' increasing the temperature and continuing the heat until all the tin alloys with the copper to form a stiff bronze skin on the plates and the wire melts and flows into the joints between succeeding plates.

5. The method of forming a spaced plate assembly comprising assembling in proper relation a series of preformed relatively thin tin coated copper plates, placing a fusible sealing material adjacent contacting surfaces of the plates and then applying and continuing heat to flow said material between said'surfaces and to concurrently alloy all the tin coating with the adjacent copper and thereby stiften the thin plates.

6. The method of bonding together a series of nested preformed and relatively thin plates and simultaneously stiflening the thin walls thereof,

comprising the nesting together of preformed tin coated copper plates, the disposition adjacent contacting surfaces of the plates of phosphorcopper wires, and the application to .the assembly of heat at approximately 1450 F., until all the tin is alloyed with the copper.

LEDRA M. LAWTON. LEROY W. BHU'ITS. 

